The present invention relates in general to a cooling fan for rotating electrical machinery such as motors and alternators, and more specifically to fan blade structures which result in reduced fan noise.
Dynamoelectric machines, such as generators, alternators, and motors, are types of rotating machinery which convert mechanical energy into electrical energy and vice versa. Such machines include a stator and a rotor. The rotor is coupled to a shaft for rotation adjacent the stator. Either the rotor or stator, or both, include windings which conduct current and thus produce heat. Other electrical components such as rectifier diodes and mechanical components such as bearings also produce heat and are often located inside the housing of the rotating machine.
In order to provide cooling of such machines, the housing is typically ventilated to allow airflow through the machine. In addition, fans may be included on the rotor or the shaft for drawing air through the housing.
An automotive alternator is known having a structure wherein a fan is attached to each end of the rotor interior of the housing. A front fan draws air in through the front end of the housing and out through the side of the housing. A rear fan draws air through the rear end of the housing and out through the side of the housing. Thus, cooling airflow can be directed to all interior parts, including electrical components, bearings, and windings.
The main disadvantage connected with the use of cooling fans is the noise generated. Since most fans, such as air conditioning fans and automobile alternators, are operated in the vicinity of people, it is desirable to minimize noise generation. Prior art attempts to solve the noise problem have failed to achieve sufficiently reduced noise with a structure which is suitable for inexpensive, large volume production. Prior art solutions typically assume a constant fan speed. That is not a valid assumption for an automobile alternator, which is required to operate over a wide speed range.
U.S. Pat. No. 4,162,419 granted to DeAngelis, discloses an automotive alternator having airflow through the alternator as a result of an internal fan rotating within the housing. Such airflow is directed over semiconductor elements within the alternator. An exterior fan is also provided on the opposite end of the alternator. The fans described in this patent provide efficient cooling of all components, but it would be desirable to decrease the noise levels produced.
U.S. Pat. No. 4,684,324, issued to Perosino, teaches a radiator fan for motor vehicles with a central hub and an outer ring which are joined by curved blades. In an attempt to provide quiet operation of the fan, a particular curvature is provided for the fan blades as viewed in the axial direction. Manipulating the blade curvature has made some beneficial impact on noise generation.
German Offenlegungsschrift 2617029 discloses a fan intended for the electrical generator of an internal combustion engine. The fan is made from a stamped metal plate such that axially projecting fan blades are produced. In addition to curved blades defining a curved flow path, the blades are given no sharp edges so as to achieve some improvement in noise performance.
In the publications Raj et al. (I), Noise Generation In FC Centrifigual Fan Rotors, Fluid Transients and Acoustics in the Power Industry, ASME Annual Winter Meeting, San Francisco, Calif. (1978), pages 289--300, and Raj et al. (II), Measurements of the Mean Flow Velocity and Velocity Fluctuations at the Exit of an FC Centrifugal Fan Rotor, Journal of Engineering for Power, Transactions of ASME, Vol. 103 (April 1981), pages 393-399, flow separation in the air flow through the fan is identified as the major source of noise. These publications propose a two-dimensional aerofoil blade shape with a convergent blade passage to reduce the flow separation and thus reduce noise. Such blade shapes cannot be produced by simple manufacturing techniques such as stamping of sheet metal parts. Furthermore, thickening of the blades undesirably adds mass to the fan which reduces the efficiency of the machine and raises the requirements for mechanical strength of the fan.